Lattice preferred orientation and seismic anisotropy of chloritoid in subduction zone

Subduction zones are often characterized by the presence of strong trench-parallel seismic anisotropy and large delay times. Hydrous minerals, owing to their large elastic anisotropy and strong lattice preferred orientations (LPOs) are often invoked to explain these observations. However, the elasticity and LPO of chloritoid, which is one such hydrous phases relevant in subduction zone settings, is poorly understood. In this study, we measured the LPO of polycrystalline chloritoid in natural rock samples and obtained the LPO-induced seismic anisotropy and evaluated the thermodynamic stability field of chloritoid in subduction zones. The LPO of chloritoid aggregates displayed a strong alignment of the [001] axes subnormal to the rock foliation, with a girdle distribution of the [100] axes and the (010) poles subparallel to the foliation. New elasticity data of single-crystal chloritoid showed a strong elastic anisotropy of chloritoid with 47% for S-waves (VS) and 22% for P-waves (VP), respectively. The combination of the LPO and the elastic anisotropy of the chloritoid aggregates produced a strong S-wave anisotropy of AVS = 18% and a P-wave anisotropy of AVP = 10%. Our results indicate that the strong LPO of chloritoid along the hydrated slab-mantle interface and in subducting slabs can influence trench-parallel seismic anisotropy in subduction zones with &#8220;cold&#8221; geotherms.

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Seismic Anisotropy in Subduction Zones: Evaluating the Role of Chloritoid

Frontiers in Earth Science ◽

10.3389/feart.2021.644958 ◽

2021 ◽

Vol 9 ◽

Author(s):

Jungjin Lee ◽

Mainak Mookherjee ◽

Taehwan Kim ◽

Haemyeong Jung ◽

Reiner Klemd

Keyword(s):

Subduction Zones ◽

Seismic Anisotropy ◽

Elastic Anisotropy ◽

P Wave ◽

Stability Field ◽

S Wave ◽

Hydrous Minerals ◽

Rock Samples ◽

Natural Rock

Subduction zones are often characterized by the presence of strong trench-parallel seismic anisotropy and large delay times. Hydrous minerals, owing to their large elastic anisotropy and strong lattice preferred orientations (LPOs), are often invoked to explain these observations. However, the elasticity and the LPO of chloritoid, which is one of such hydrous phases relevant in subduction zone settings, are poorly understood. In this study, we measured the LPO of polycrystalline chloritoid in natural rock samples, obtained the LPO-induced seismic anisotropy, and evaluated the thermodynamic stability field of chloritoid in subduction zones. The LPO of chloritoid aggregates displayed a strong alignment of the [001] axes subnormal to the rock foliation, with a girdle distribution of the [100] axes and the (010) poles subparallel to the foliation. New elasticity data of single-crystal chloritoid showed a strong elastic anisotropy of chloritoid with 47% for S-waves (VS) and 22% for P-waves (VP), respectively. The combination of the LPO and the elastic anisotropy of the chloritoid aggregates produced a strong S-wave anisotropy with a maximum AVS of 18% and a P-wave anisotropy with an AVP of 10%. The role of chloritoid LPO in seismic anisotropy was evaluated in natural rock samples and a hypothetical blueschist. Our results indicate that the strong LPO of chloritoid along the subduction interface and in subducting slabs can influence the trench-parallel seismic anisotropy in subduction zones with “cold” geotherms.

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Twin induced attenuation of seismic anisotropy in lawsonite blueschist

10.5194/egusphere-egu21-6954 ◽

2021 ◽

Author(s):

Seungsoon Choi ◽

Olivier Fabbri ◽

Gültekin Topuz ◽

Aral Okay ◽

Haemyeong Jung

Keyword(s):

Electron Microscope ◽

Oceanic Crust ◽

Subduction Zones ◽

Seismic Anisotropy ◽

Elastic Anisotropy ◽

S Wave ◽

Electron Backscattered Diffraction ◽

Crystal Preferred Orientation ◽

Fabric Strength ◽

Scanning Electron

Lawsonite is an important mineral to understand seismic anisotropy in subducting oceanic crust because of its large elastic anisotropy and prevalence in cold subduction zones. However, there is a lack of knowledge on how lawsonite twinning affects seismic anisotropy despite previous reports showing the existence of twins in lawsonite. We thus investigated the effect of twins in lawsonite on crystal preferred orientation (CPO), fabric strength, and seismic anisotropy of lawsonite using the lawsonite blueschists from Alpine Corsica (France) and Sivrihisar Massif (Turkey). CPOs of minerals were measured by using the electron backscattered diffraction (EBSD) facility attached to scanning electron microscope. The EBSD analyses of lawsonite revealed that {110} twin in lawsonite is developed and [001] axes are strongly aligned subnormal to the foliation and both [100] and [010] axes are aligned subparallel to the foliation. It is found that the existence of twins in lawsonite could induce a large attenuation of seismic anisotropy, especially for the maximum S-wave anisotropy up to 18.4 % in lawsonite and 24.3 % in the whole rocks. Therefore, lawsonite twinning needs to be considered in the interpretation of seismic anisotropy in the subducting oceanic crust in cold subduction zones.

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New Imaging Approach for Constraining the Azimuth and Dip of Seismic Anisotropy Using Teleseismic P-wave Delays and its Application to the Western United States

10.5194/egusphere-egu21-14514 ◽

2021 ◽

Author(s):

Brandon VanderBeek ◽

Miles Bodmer ◽

Manuele Faccenda

Keyword(s):

United States ◽

Subduction Zones ◽

Seismic Anisotropy ◽

Elastic Anisotropy ◽

P Wave ◽

Western United States ◽

Anisotropic Structure ◽

Low Velocity ◽

Mantle Mineral ◽

Juan De Fuca

Despite the well-established anisotropic nature of Earth&#8217;s upper mantle, the influence of elastic anisotropy on teleseismic P-wave imaging remains largely ignored. Unmodeled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional and thermal heterogeneities. Here, we present a new parameterization for imaging arbitrarily oriented hexagonal anisotropy using teleseismic P-wave delays. We evaluate our tomography algorithm by reconstructing geodynamic simulations of subduction that include predictions for mantle mineral fabrics. Our synthetic tests demonstrate that accounting for both the dip and azimuth of anisotropy in the inversion is critical to the accurate recovery of both isotropic and anisotropic structure. We then perform anisotropic inversions using data collected across the western United States and offshore Cascadia. Our preliminary models show a clear circular pattern in the azimuth of anisotropy around the southern edge of the Juan de Fuca slab that is remarkably similar to the toroidal flow pattern inferred from SKS splits. We also image dipping anisotropic domains coincident with the descending Juan de Fuca slab. In contrast to prior isotropic tomographic results, the Juan de Fuca slab in our anisotropic model is characterized by more uniform P-wave speeds and is without an obvious slab hole below ~150 km depth. We also find a general decrease in the magnitude of mantle low-velocity zones throughout the model relative to prior studies. These results highlight the sensitivity of teleseismic P-waves to anisotropic structure and the importance of accounting for anisotropic heterogeneity in the imaging of subduction zones.

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The rheology of talc at high P-T conditions with implications for subduction-zone dynamics

10.5194/egusphere-egu2020-12696 ◽

2020 ◽

Author(s):

Yuval Boneh ◽

Matej Pec ◽

Greg Hirth

Keyword(s):

Subduction Zone ◽

Subduction Zones ◽

Mantle Wedge ◽

Microstructural Analysis ◽

Stability Field ◽

Localized Deformation ◽

Hydrous Minerals ◽

Regional Seismicity ◽

Pressure Threshold ◽

Confining Pressures

Subduction-zone dynamics, kinematics, and seismicity are strongly affected by the rheology of hydrous phyllosilicates. Although there is growing evidence for hydrous minerals in the subducting plate, mantle wedge, and the interface between the plates, we are continuing to learn more about the rheological behavior of phyllosilicates at the relevant pressures. Talc is stable to depths of &#8776;100 km and has been found in fault rocks and subduction-zones m&#233;langes as the product of metasomatism and/or mineral breakdown (e.g., breakdown of antigorite). The frictional strength of talc under low to intermediate pressures (up to ~400 MPa) was studied and demonstrated some of the mineral&#8217;s unique rheology; however, there is a lack of data for pressures of P > 0.5 GPa. Here we present the first rheological and microstructural analysis of experimentally deformed talc under pressure and temperature conditions relevant for the rheology of a subducted slab or mantle wedge.We analyzed the mechanical and microstructural evolution of 15 samples of natural talc cylinders deformed using a high P-T deformation &#8216;Griggs&#8217; type apparatus. We used natural samples comprise of >98 % talc and analyzed the post-mortem microstructure and chemistry of the samples using optical microscopy, scanning electron microscopy, and electron microprobe. The experiments were performed at confining pressures from 0.5 to 2 GPa and temperatures of 25 to 700&#176;C; all within the talc stability field. Results show that the strength of talc at 25&#176;C or 400&#176;C is pressure-dependent up to the highest pressure tested (2 GPa). This behavior is attributed to brittle/semi-brittle mechanisms. At higher temperatures (500-700&#176; C) and above a pressure threshold the strength becomes independent of pressure (e.g., when P > 1 GPa at T = 600 &#176; C), indicating that dilatant cracking is suppressed at these pressures. However, microstructural analysis indicates that fracturing is evident in all samples at all conditions examined. Interestingly, samples deformed at higher temperatures (>600&#176;C) show more localized deformation. A synthesis of results from this study and previously published studies demonstrate that the strength of talc only becomes temperature-dependent at higher pressures. It is suggested that an increasing P-T geotherm of a subducted slab is likely to induce weakening and localization of talc-rich layers with possible implications for the mechanism to induce/hinder regional seismicity and affect the plate-coupling between the subducted and riding plates.&#160;&#160;&#160;

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Twin Induced Reduction of Seismic Anisotropy in Lawsonite Blueschist

Minerals ◽

10.3390/min11040399 ◽

2021 ◽

Vol 11 (4) ◽

pp. 399

Author(s):

Seungsoon Choi ◽

Olivier Fabbri ◽

Gültekin Topuz ◽

Aral I. Okay ◽

Haemyeong Jung

Keyword(s):

Oceanic Crust ◽

Subduction Zones ◽

Seismic Anisotropy ◽

Elastic Anisotropy ◽

Electron Backscatter Diffraction ◽

S Wave ◽

Crystal Preferred Orientation ◽

Electron Backscatter ◽

Backscatter Diffraction ◽

Insufficient Knowledge

Lawsonite is an important mineral for understanding seismic anisotropy in subducting oceanic crust due to its large elastic anisotropy and prevalence in cold subduction zones. However, there is insufficient knowledge of how lawsonite twinning affects seismic anisotropy, despite previous studies demonstrating the presence of twins in lawsonite. This study investigated the effect of lawsonite twinning on the crystal preferred orientation (CPO), CPO strength, and seismic anisotropy using lawsonite blueschists from Alpine Corsica (France) and the Sivrihisar Massif (Turkey). The CPOs of the minerals are measured with an electron backscatter diffraction instrument attached to a scanning electron microscope. The electron backscatter diffraction analyses of lawsonite reveal that the {110} twin in lawsonite is developed, the [001] axes are strongly aligned subnormal to the foliation, and both the [100] and [010] axes are aligned subparallel to the foliation. It is concluded that the existence of twins in lawsonite could induce substantial seismic anisotropy reduction, particularly for the maximum S-wave anisotropy in lawsonite and whole rocks by up to 3.67% and 1.46%, respectively. Lawsonite twinning needs to be considered when determining seismic anisotropy in the subducting oceanic crust in cold subduction zones.

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Numerical simulation of azimuthal acoustic logging in a borehole penetrating a rock formation boundary

Geophysics ◽

10.1190/geo2015-0265.1 ◽

2016 ◽

Vol 81 (3) ◽

pp. D283-D291 ◽

Cited By ~ 4

Author(s):

Peng Liu ◽

Wenxiao Qiao ◽

Xiaohua Che ◽

Xiaodong Ju ◽

Junqiang Lu ◽

...

Keyword(s):

Domain Model ◽

P Wave ◽

S Wave ◽

Angle Variation ◽

P Waves ◽

Acoustic Logging ◽

S Waves ◽

Rock Formation ◽

Logging Tool ◽

Difference Time

We have developed a new 3D acoustic logging tool (3DAC). To examine the azimuthal resolution of 3DAC, we have evaluated a 3D finite-difference time-domain model to simulate a case in which the borehole penetrated a rock formation boundary when the tool worked at the azimuthal-transmitting-azimuthal-receiving mode. The results indicated that there were two types of P-waves with different slowness in waveforms: the P-wave of the harder rock (P1) and the P-wave of the softer rock (P2). The P1-wave can be observed in each azimuthal receiver, but the P2-wave appears only in the azimuthal receivers toward the softer rock. When these two types of rock are both fast formations, two types of S-waves also exist, and they have better azimuthal sensitivity compared with P-waves. The S-wave of the harder rock (S1) appears only in receivers toward the harder rock, and the S-wave of the softer rock (S2) appears only in receivers toward the softer rock. A model was simulated in which the boundary between shale and sand penetrated the borehole but not the borehole axis. The P-wave of shale and the S-wave of sand are azimuthally sensitive to the azimuth angle variation of two formations. In addition, waveforms obtained from 3DAC working at the monopole-transmitting-azimuthal-receiving mode indicate that the corresponding P-waves and S-waves are azimuthally sensitive, too. Finally, we have developed a field example of 3DAC to support our simulation results: The azimuthal variation of the P-wave slowness was observed and can thus be used to reflect the azimuthal heterogeneity of formations.

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The 1998 Valhall microseismic data set: An integrated study of relocated sources, seismic multiplets, and S-wave splitting

Geophysics ◽

10.1190/1.3205028 ◽

2009 ◽

Vol 74 (5) ◽

pp. B183-B195 ◽

Cited By ~ 40

Author(s):

K. De Meersman ◽

J.-M. Kendall ◽

M. van der Baan

Keyword(s):

Seismic Anisotropy ◽

Amplitude Ratio ◽

Temporal Variations ◽

Oil Field ◽

P Wave ◽

Wave Form ◽

S Wave ◽

Data Set ◽

Wave Splitting ◽

Source Mechanisms

We relocate 303 microseismic events recorded in 1998 by sensors in a single borehole in the North Sea Valhall oil field. A semiautomated array analysis method repicks the P- and S-wave arrival times and P-wave polarizations, which are needed to locate these events. The relocated sources are confined predominantly to a [Formula: see text]-thick zone just above the reservoir, and location uncertainties are half those of previous efforts. Multiplet analysis identifies 40 multiplet groups, which include 208 of the 303 events. The largest group contains 24 events, and five groups contain 10 or more events. Within each multiplet group, we further improve arrival-time picking through crosscorrelation, which enhances the relative accuracy of the relocated events and reveals that more than 99% of the seismic activity lies spatially in three distinct clusters. The spatial distribution of events and wave-form similarities reveal two faultlike structures that match well with north-northwest–south-southeast-trending fault planes interpreted from 3D surface seismic data. Most waveform differences between multiplet groups located on these faults can be attributed to S-wave phase content and polarity or P-to-S amplitude ratio. The range in P-to-S amplitude ratios observed on the faults is explained best in terms of varying source mechanisms. We also find a correlation between multiplet groups and temporal variations in seismic anisotropy, as revealed by S-wave splitting analysis. We explain these findings in the context of a cyclic recharge and dissipation of cap-rock stresses in response to production-driven compaction of the underlying oil reservoir. The cyclic nature of this mechanism drives the short-term variations in seismic anisotropy and the reactivation of microseismic source mechanisms over time.

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On the Green function of the Lord–Shulman thermoelasticity equations

Geophysical Journal International ◽

10.1093/gji/ggz453 ◽

2019 ◽

Vol 220 (1) ◽

pp. 393-403 ◽

Cited By ~ 5

Author(s):

Zhi-Wei Wang ◽

Li-Yun Fu ◽

Jia Wei ◽

Wanting Hou ◽

Jing Ba ◽

...

Keyword(s):

Green’S Function ◽

Green's Function ◽

Parabolic Type ◽

Second Order ◽

P Wave ◽

Thermal Mode ◽

S Wave ◽

Order Tensor ◽

P Waves ◽

Thermal Conductivities

SUMMARY Thermoelasticity extends the classical elastic theory by coupling the fields of particle displacement and temperature. The classical theory of thermoelasticity, based on a parabolic-type heat-conduction equation, is characteristic of an unphysical behaviour of thermoelastic waves with discontinuities and infinite velocities as a function of frequency. A better physical system of equations incorporates a relaxation term into the heat equation; the equations predict three propagation modes, namely, a fast P wave (E wave), a slow thermal P wave (T wave), and a shear wave (S wave). We formulate a second-order tensor Green's function based on the Fourier transform of the thermodynamic equations. It is the displacement–temperature solution to a point (elastic or heat) source. The snapshots, obtained with the derived second-order tensor Green's function, show that the elastic and thermal P modes are dispersive and lossy, which is confirmed by a plane-wave analysis. These modes have similar characteristics of the fast and slow P waves of poroelasticity. Particularly, the thermal mode is diffusive at low thermal conductivities and becomes wave-like for high thermal conductivities.

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Strain-Induced Fabric Transition of Chlorite and Implications for Seismic Anisotropy in Subduction Zones

Minerals ◽

10.3390/min10060503 ◽

2020 ◽

Vol 10 (6) ◽

pp. 503

Author(s):

Dohyun Kim ◽

Haemyeong Jung ◽

Jungjin Lee

Keyword(s):

Shear Strain ◽

Subduction Zones ◽

Seismic Anisotropy ◽

Shear Plane ◽

Shear Direction ◽

S Wave ◽

Lattice Preferred Orientation ◽

Back Arc

Seismic anisotropy of S-wave, trench-parallel or trench-normal polarization direction of fast S-wave, has been observed in the fore-arc and back-arc regions of subduction zones. Lattice preferred orientation (LPO) of elastically anisotropic chlorite has been suggested as one of the major causes of seismic anisotropy in subduction zones. However, there are two different LPOs of chlorite reported based on the previous studies of natural chlorite peridotites, which can produce different expression of seismic anisotropy. The mechanism for causing the two different LPOs of chlorite is not known. Therefore, we conducted deformation experiments of chlorite peridotite under high pressure–temperature conditions (P = 0.5–2.5 GPa, T = 540–720 °C). We found that two different chlorite LPOs were developed depending on the magnitude of shear strain. The type-1 chlorite LPO is characterized by the [001] axes aligned subnormal to the shear plane, and the type-2 chlorite LPO is characterized by a girdle distribution of the [001] axes subnormal to the shear direction. The type-1 chlorite LPO developed under low shear strain (γ ≤ 3.1 ± 0.3), producing trench-parallel seismic anisotropy. The type-2 chlorite LPO developed under high shear strain (γ ≥ 5.1 ± 1.5), producing trench-normal seismic anisotropy. The anisotropy of S-wave velocity (AVs) of chlorite was very strong up to AVs = 48.7% so that anomalous seismic anisotropy in subduction zones can be influenced by the chlorite LPOs.

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Modeling guided waves to constrain the velocity structure of the oceanic crust in the subduction zone of eastern Alaska

10.5194/egusphere-egu2020-3268 ◽

2020 ◽

Author(s):

Xiaoyu Guan ◽

Yuanze Zhou ◽

Takashi Furumura

Keyword(s):

Subduction Zone ◽

Oceanic Crust ◽

High Frequency ◽

Subduction Zones ◽

Guided Waves ◽

Velocity Structure ◽

Guided Wave ◽

S Wave ◽

Arrival Times ◽

Velocity Models

Fitting subduction zone guided waves with synthetics is an ideal choice for studying the velocity structure of the oceanic crust. After an earthquake occurs in subduction zones, seismic waves can be trapped in the low-velocity oceanic crust and propagated as guided waves. The arrival time and frequency characteristics of the guided waves can be used to image the velocity structure of the oceanic crust. The analysis and modeling based on guided wave observations provide a rare opportunity to understand the velocity structure of the oceanic crust and the variations in oceanic crustal materials during the subduction process.High-frequency guided waves have been observed in the subduction zone of eastern Alaska. On several sections, observed seismograms recorded by seismic stations show low-frequency (<2Hz) onsets ahead of the main high-frequency (>2Hz) guided waves. Differences in the arrival times and dispersion characteristics of seismic phases are related to the velocity structure of the oceanic crust, and the characteristics of coda waves are related to the distribution of elongated scatters in the oceanic crust. Through fitting the observed broadband waveforms and synthetics modeled with the 2-D FDM (Finite Difference Method), we obtain the preferred oceanic crustal velocity models for several sections in the subduction zone of eastern Alaska. The preferred models can explain the seismic phase arrival times, dispersions, and coda characteristics in the observed waveforms. With the obtained P- and S- wave models of velocity structures on several sections, the material compositions they represent are deduced, and the variations of oceanic crustal materials during subducting can be understood. This provides new evidence for studying the details of the subduction process in the subduction zone of eastern Alaska.

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